The properties of a cellular solid are sensitive to both the topology of the cell material and its properties. For manufacturers, the main obstacle to obtaining superior properties has involved gaining good control over the distribution of material at the cell level in a cost effective way. As such, the most common and least expensive synthetic cellular solids remain stochastic in nature; made by variants of foaming in the liquid, solid or semi-solid state [1]. Other methods involve the solidification of liquids containing dissolved gases, bonding of hollow spheres, vapor deposition onto sacrificial templates, or investment casting using a stochastic cellular structure mold. These types of manufacturing approaches lead to cellular architectures with open, closed or mixed types of porosity.
Foaming results in cell architectures that are predominantly closed cell, often with wide distributions of cell size and many imperfections. Closed cell stochastic foams are used for sound attenuation and impact energy absorption. Open cell stochastic foams can be made using reticulated polymer foam templates. In one approach, the template is used as the pattern for an investment casting mold which is then filled with a liquid (e.g. molten metal) and solidified. In others, a vapor or fine powder slurry is deposited directly on to the template. In the latter, a subsequent heat treatment removes the organic compounds and densifies the structure. Open cell stochastic metal foams are used for lightweight heat exchangers and as the electrodes in nickel metal hydride batteries. However, their utility as load bearing structures is substantially reduced with decreasing relative density as the Young's and shear moduli along with the tensile, compressive and shear yield strengths degrade in a non-linear way (owing to ligament failure in bending). This is also true for the closed cell stochastic foams. Nonetheless, these stochastic cellular materials still look to be structurally competitive when used as the cores of sandwich panels (especially in biaxial loading) [7].
Finite-element analysis of structural configurations that give the highest weight specific stiffness lead to a truss-like cellular structure when the solid volume fraction is small. Researchers in the field of cellular solids have now begun to concentrate on an open periodic cell lattice (trusses). Small polymeric, ceramic and metallic trusses of this type are currently manufactured using rapid proto-typing or injection molding techniques. For example, by fabricating a polymer or wax pattern of the appropriate truss architecture, miniature metal trusses follow by investment casting. The resulting structures are known as lattice block or truss materials. Individual cells can be small (a few mm). By manipulating the truss architecture, properties can be widely modified. Like proven truss designs, the Young's and shear moduli along with the tensile, compressive and shear yield strengths of these materials vary with relative density in a linear way (trusses are in tension/compression with no bending). This becomes especially important at low relative density where properties far exceed those of stochastic cellular solids. These are just a few of the benefits to be gained when good control over the distribution of material at the cell level is achieved.
However, the casting approaches used to manufacture miniature trusses are expensive and the resulting structures are subject to large (2-3) knockdown by casting factors (e.g., entrapped porosity, shrinkage residual stress, etc.). Furthermore, many materials of potential interest are difficult to cast and do not favorably respond to post-processing (e.g. heat treatment).
Moreover, both stochastic and periodic cellular metals have attracted interest as alternatives to honeycomb when used as the cores of sandwich structures designed to support in-plane compressive or bending loads [1]. For successful implementation, these cellular metal based approaches must compete against established panel stiffening and strengthening concepts. Conventional panel stiffening involves the attachment of stringers that increase the polar and second moment of cross-section area with modest added weight [1]. Panels of this type are often made by machining stiffeners from thick blanks and fastening to a sheet. When fabricated in this way, the panels can be quite light and stiff however, they also show substantial anisotropy in the bending plane and are relatively expensive due to the poor utilization of material and high machining cost.
Other ways to stiffen a panel involve waffling or sandwich construction [1-4]. For the latter, thin strong skins are bonded to the sides of a lightweight core 3 as shown in FIG. 1. Like the flanges of an I-beam, the skins 4 provide support in bending with one skin in compression and the other in tension. The core functions in a manner similar to the web of an I-beam. That is, it resists shear and compressive loads while separating the skins far apart to generate a high second moment cross-section area and therefore high rigidity.
Honeycomb core sandwich structures 2 are the current state-of-the-art choice for weight sensitive applications such as aircraft and satellite structures [2]. But there are difficulties with forming them into complex (non-planar) shapes due to induced anticlastic curvature [2]. Also, the closed nature of the porosity can trap moisture leading to corrosion. In space applications, their skins are susceptible to interfacial debonding.
Open cell cores based upon tetrahedral truss concepts [5,6] allow fluids to readily pass through which could make them less susceptible to internal corrosion and depressurization induced delamination. When used as sandwich cores, they are more amenable to shaping into complex shapes. They are also attractive for multifunctional applications such as cross flow heat exchangers due to the interconnected nature of the porosity [1].
Multifunctional materials designers seek to tailor load support properties of interest (e.g. stiffness and strength) in the most efficient way through adjustment of the open cell topology, relative density and material type. The intervening space can then be used for other functionalities [7]. For example, the porosity within a load supporting cellular metal structure could also be used to simultaneously enhance impact/blast energy absorption [8,9], noise attenuation [8], catalytic activity [8], filtration efficiency [8], electrical energy storage [10] or act as the host for the in-growth of biological tissue [11]. Stochastic open cell foams have been proposed for sandwich structure cores but their mechanical properties are inferior to honeycomb [1]. FIG. 2 is a graphical representation that summarizes the Young's and shear moduli relative density relationships for various cellular concepts.
The elastic moduli of stochastic open cell foams are considerably lower than those of regular hexagonal honeycomb at low relative density. Similar trends are seen with the yield strength. These differences are a consequence of ligament bending [12]. For improved core performance, cellular topologies that deform by means of ligament stretching or compressing are preferred [7]. A prototypical example is the tetrahedral truss sandwich core [13] made by investment casting. However, high quality structures of this type are difficult to fabricate in miniature size at acceptable cost.
There exist a need in the art for methods for making multifunctional truss-based periodic cellular solids that are near the theoretical maximum stiffness and strength for a cellular solid, yet is characterized by low production costs. Accordingly, the present invention truss-based cellular solids provides a host of new and interesting multifunctional structures that could be made. The present invention provides cost effective ways of making high quality truss-based cellular solids of this type that overcomes many obstacles of the prior art. The present inventors have recently suggested a textile-based approach—as shown in pending co-assigned PCT International Application No. PCT/US01/17363, entitled “Multifunctional Periodic Cellular Solids And The Method Of Making Thereof,” filed on May 29, 2001, of which is hereby incorporated by reference herein—and now provide with the present invention another way that includes closed and mixed types of porosity along with the open type. Metals, ceramics, glasses, polymers, composites and even semiconductors can all be fabricated by the present invention method. For example, for metals there is provided a perforation and deformation process followed by transient liquid phase bonding. With the new approach, miniature truss-like structures with exceptional strength/weight ratios for multifunctional structural applications are readily made at acceptable cost. The cores of these structures are bonded to thin metal facesheets using the transient liquid phase approach. These structures can be planar or curved and can be bonded to themselves, facesheets or other structures using the transient liquid phase or other bonding approaches.